7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Role of isotopes in microturbulence from linear to saturated Ohmic confinement
regimesLei Qi, Jae-Min Kwon, T. S. Hahm, M. Leconte, Sumin Yi, Y. W. Cho and Janghoon Seo
Korea Institute of Fusion Energy, Daejeon 169-148, South Korea
e-mail (speaker): qileister@kfe.re.kr
Deuterium (D) and tritium (T), pivotal to fusion energy,
serve as the fuel for power generation in fusion plants,
making their reactor-level operation a crucial objectivefor ITER[1]. Various tokamak experiments have shownthat isotopes enhance energy confinement, characterized
by a scaling 𝜏𝐸 ∝ 𝑀𝑖𝜎 , where 𝜏𝐸 denotes energy
confinement time, 𝑀𝑖 represents the isotope mass ratio
to hydrogen, and 𝜎 varies approximately between 0.2
and 0.5[2,3,4] in nearly all tokamak operation regimes.
Notably, isotopic dependence in Ohmic plasmas on
ASDEX displayed 𝜎 = 0.31 in low density linear
Ohmic confinement (LOC) regime and 𝜎 = 0.5 in highdensity saturated Ohmic confinement (SOC) regime[2].
The empirical scaling laws for isotope effects haveproduced 𝜎 = 0.5 as per the ITER L-mode power
scaling ITER89-P[5] and 𝜎 = 0.2 according to theITER H-mode confinement scaling ITER-IPB(y)[3].
Further, recent experiments have shed light on theinfluence of isotopes on the L-H power threshold inDIII-D[6], as well as on the pedestal height in JET withan ITER-like wall[7] and ASDEX Upgrade[8]. Given
these findings, it is pivotal to unravel the effects ofisotopes in less complex plasma states like Ohmic andL-mode.
The first principle gyrokinetic numerical experiments
investigating the isotopic dependence of energy
confinement achieve a quantitative agreement withexperimental empirical scalings, particularly in Ohmic
and L-mode tokamak plasmas. Figure 1 compares thenumerical results (Fig.1a) with the experimentalmeasurements (Fig.1b) in ASDEX, indicating anexcellent agreement in the trend. In addition, the scaling
factor 𝜎 from the numerical experiments shows aquantitative consensus with the empirical scalings fromASDEX[2],ITER89-P[5]forL-modeandITER-IPB(y)[3] for H-mode, as illustrated in Table I.
Mitigation of turbulence radial electric field intensity|𝛿𝐸𝑟 |2 and associated poloidal 𝜹𝑬𝒓 × 𝑩 fluctuating
velocity with the radial correlation length 𝑙𝑐𝑟 ∝ 𝑀𝑖0.11
strongly deviating from the gyro-Bohm scaling is
identified as the principal mechanism behind the isotope
effects. Three primary contributors are classified, thedeviation from gyro-Bohm scaling, zonal flow andtrapped electron turbulence stabilization. Contributions
from these three mechanisms are also quantified asshown in Figure 2. Zonal flow enhances isotope effectsprimarily through reinforcing the inverse dependence ofturbulence decorrelation rate on isotope mass with 𝜔𝑐 ∝𝑀𝑖−0.76 , which markedly differs from the characteristic
linear frequency. The findings offer new insights into
isotope effects, providing critical implications for energy
confinement optimization in tokamak plasmas.
Figure 1: (a) Effective
thermalenergyconfinement time 𝜏𝑒𝑓𝑓 isdepicted as a function ofdensity $n$ for cases withzonal flows (w ZF) andwithout zonal flows (woZF), and for H, D and T.
(b) Energy confinement
time as a function ofdensity
adaptedfrom
Fig.1bin
Ref:M.
Bessenrodt-Weberpals, F.
Wagner, ASDEX TEAM,
Nuclear Fusion, Vol. 33,
No. 8 (1993).

Figure 2: Contributions to thescaling factor 𝜎 in theisotopic
dependenceofenergy confinement scaling𝜏𝑒𝑓𝑓 ∝ 𝑀𝑖𝜎 from the threeparticipators, deviation fromgyro-Bohm scaling (green),
TEM stabilization (blue) andzonal flow (red) in both ITG
dominant and TEM dominant
regimes are presented.

References[1] B. Bigot, Nucl. Fusion 59, 112001 (2019)[2] M. Bessenrodt-Weberpals et al., Nucl. Fusion 33,1205 (1993)[3] ITER Physics Expert Group on Confinement andTransport, Nucl. Fusion 39, 2175 (1999)[4] K. Ida, Rev. Mod. Plasma Phys. 7, 23 (2023)[5] P. N. Yushmanov et al., Nucl. Fusion 30, 1999 (1990)[6] Z. Yan et al., Nucl. Fusion 57, 126015 (2017)[7] C. F. Maggi et al., Nucl. Fusion 59, 076028 (2019)[8] P. A. Schneider et al., Plasma Phys. Control. Fusion
63, 064006 (2021)